Conventional aromatic polycarbodiimides are obtained by using diphenylmethane diisocynate (MDI), tolylene diisocyanate (TDI), etc., as a monomer, and polymerizing this. Such a polycarbodiimide is used as a flameproofing film and a heat resistant adhesive due to its excellent heat resistance.
However, such a conventional polycarbodiimide film is poor in the solubility, is gelled and deposited as solids with increasing its molecular weight, whereby a sufficiently high molecular weight polymer is not obtained. Further, the polycarbodiimide film has a heat resistance in the point that even when the film is exposed to high temperature of 400.degree. C. or more, a volatile gas and decomposed monomers are not formed, but when the film is heat-treated at a temperature of 200.degree. C. or higher, the film loses the self-supporting property and becomes brittle, whereby the film cannot endure the practical use. Furthermore, when the film is heat-adhered to an adherend such as a copper foil, etc., it cannot say that the adhesive force is sufficient.